CMOS compatible integrated pressure sensor

Abstract

A pressure sensor fabricated onto a substrate using conventional CMOS fabrication processes. The pressure sensor is built on a substrate having a first conductivity type and has defined in it a well of an opposite conductivity type. This well defines a membrane. Resistors are diffused into the well. Source/drain regions are provided for leadouts for the resistors. An n-cap is provided for the resistors. Metalization contacts may be provided to connect the membrane to a positive bias during a membrane etching process. A cavity is provided on the underside of the substrate through which pressure is applied to the membrane. Signal conditioning circuitry, such as an operational amplifier, may also be fabricated on the same substrate preferably using the same IC processes.

We claim:  
     
       1. A micromachined pressure sensor, comprising: 
       a. a substrate having a first conductivity type and defining a cavity on a first side;  
       b. a well having a second, opposite conductivity type imbedded into a second side of the substrate and in contact with the cavity, the well defining a membrane, where said well directly contacts said substrate;  
       c. at least one resistor defined in the membrane;  
       d. a source and a drain region connected to the at least one resistor; and  
       e. a cap having the second conductivity type on a surface of the at least one resistor.  
     
     
       2. The sensor of claim  1 , further comprising metal contacts connected to the membrane. 
     
     
       3. The sensor of claim  2 , wherein the metal contacts are configured to connect to a bias during a membrane etch process. 
     
     
       4. The sensor of claim  1 , further including a signal conditioning circuit connected to the at least one resistor. 
     
     
       5. The sensor of claim  4 , wherein the signal conditioning circuit is integrated on the substrate. 
     
     
       6. The sensor of claim  4 , wherein the signal conditioning circuit is an operational amplifier. 
     
     
       7. The sensor of claim  4 , wherein the signal conditioning circuit is an analog operational amplifier. 
     
     
       8. The sensor of claim  1 , wherein the at least one resistor is one of four resistors. 
     
     
       9. The sensor of claim  8 , wherein two resistors are oriented parallel to a principal stress of the membrane and two resistors are oriented perpendicularly to the principal stress of the membrane. 
     
     
       10. The sensor of claim  1 , wherein the source and drain regions are leadouts for the at least one resistor. 
     
     
       11. The sensor of claim  1 , wherein the cap is configured to shield the at least one resistor from outside charges. 
     
     
       12. The sensor of claim  1 , wherein the cap is configured to eliminate depletion effects. 
     
     
       13. The sensor of claim  1 , wherein the at least one resistor is one of four resistors arranged as a Wheatstone bridge and having a signal conditioning circuit connected thereto. 
     
     
       14. The sensor of claim  13 , wherein pressure may be detected according to:            V   out       V   in       ≅       1   2          π   44        σ                     
       where: 
       π 44 =piezoresistive coefficient;  
       σ=principal stress;  
       V IN =a voltage applied to the Wheatstone bridge; and  
       V OUT =a voltage output by the signal conditioning circuit.